1. Field of the Invention
The invention relates to an exhaust valve structure provided in an exhaust passage of an internal combustion engine equipped for a vehicle.
2. Description of the Related Art
In general, an exhaust valve structure of this type is provided in an exhaust passage of an internal combustion engine for the purpose of changing the flow passage of exhaust gas, controlling the flow rate of exhaust gas, or the like.
In an existing art, such an exhaust valve structure is applied to an exhaust heat recovery system that recovers the heat of exhaust gas emitted from an engine serving as an internal combustion engine through heat exchange with engine coolant to utilize the heat for heating, facilitating the warm-up of the engine, or the like (for example, see Japanese Patent Application Publication No. 2008-101479 (JP-A-2008-101479)).
The exhaust heat recovery system described in JP-A-2008-101479 is provided in a bypass passage for bypassing an exhaust system heat exchanger. The exhaust heat recovery system opens or closes the bypass passage in accordance with the pressure of exhaust gas to thereby switch between an exhaust heat recovery mode in which exhaust gas exchanges heat with the engine and a normal mode in which exhaust gas passes through the bypass passage.
The exhaust valve structure that constitutes the exhaust heat recovery system includes a substantially rectangular box-shaped valve housing and a valve. The valve housing is connected to an inner pipe having a bypass passage inside, and the internal space of the valve housing serves as a downstream end of the bypass passage. The valve is pivotably attached to the valve housing via a pivot shaft. The valve housing has a pair of main exhaust ports at respective side walls arranged along a direction in which exhaust gas flows. The pair of main exhaust ports are opposite to each other in a direction perpendicular to the direction in which exhaust gas flows. The valve housing has a sub-exhaust port at the lower portion of an end plate that closes the downstream end in the direction in which exhaust gas flows. The sub-exhaust port has an area smaller than the sectional area of an exhaust gas passage. In addition, the valve includes a main valve portion for opening or closing the pair of main exhaust ports and a sub-valve portion for opening or closing the sub-exhaust port.
In the thus configured exhaust valve structure described in JP-A-2008-101479, a return spring is anchored to one end of the pivot shaft of the valve, and the valve is placed in a closed position in which the main valve portions and the sub-valve portion respectively close the main exhaust valve ports and the sub-exhaust valve port by the urging force of the return spring. On the other hand, when the pressure of exhaust gas exerted on the valve is higher than or equal to a predetermined value, the valve pivots against the urging force of the return spring to shift from the closed position to an open position in which the main exhaust ports and the sub-exhaust port are open.
Then, in the exhaust valve structure, exhaust pulse that propagates through the bypass passage is uniformly exerted on the pair of main valve portions, so the force based on the exhaust pulse exerted on one of the main valve portions becomes the reaction force against the force based on the exhaust pulse exerted on the other one of the main valve portions. Therefore, the forces for displacing the valve with respect to the valve housing owing to exhaust pulse are cancelled. As a result, in the exhaust valve structure, chattering of the valve due to exhaust pulse exerted on the pair of main valve portions is suppressed.
In addition, in the above described exhaust valve structure, the pressure receiving area of the sub-valve portion on which exhaust pulse is exerted together with the main valve portions is small because of the small-area sub-exhaust port, so the sub-valve portion may be formed to be insensitive to exhaust pulse. Therefore, chattering of the valve due to exhaust pulse exerted on the sub-valve portion is suppressed.
In addition, in the exhaust valve structure described in JP-A-2008-101479, a wire mesh is interposed between the peripheral portion of each main exhaust port of the valve housing and a corresponding one of the main valve portions of the valve to serve as a seal to thereby prevent leakage of exhaust gas.
However, in the above described existing exhaust valve structure described in JP-A-2008-101479, chattering of the valve is suppressed to suppress noise due to the chattering; however, chattering is not reliably prevented, and there is a problem that it is impossible to suppress noise when chattering occurs.
In addition, in the existing exhaust valve structure, the wire mesh is interposed between the valve housing and the valve; however, the wire mesh is provided so as to serve as a seal, and the wire mesh is not intended to absorb an impact during chattering to suppress an impact sound.
Therefore, if the wire mesh is just interposed between the valve housing and the valve as in the case of the existing art, an impact during chattering cannot be absorbed, and, as a result, there is a problem that it is insufficient to prevent noise resulting from chattering.